Do nothing if converting to the same type of Note

Do nothing if converting to the same type of Note

The advantage is that the buffer will no longer be
marked as dirty when converting to the same type.

A quick break here is done instead of disabling the
LFUN because disabling the LFUN would lead to a greyed out
entry, which might confuse users. In the future, we might
want to have a radio button for switching between notes.